This invention relates to a method of preparing trifluoromethylbenzonitrile (abbreviated to TFMBN) from trifluoromethylbenzaldehyde.
TFMBN is useful as an intermediate material of some medicines including tranquilizer and agricultural chemicals and also as an intermediate material of a certain type of liquid crystal.
U.S. Pat. No. 4,211,721 shows obtaining TFMBN by cyanizing chlorobenzotrifluoride, but this method is not suited to industrial practice because of using a highly toxic cyanide. DE-A No. 2,214,061 shows preparing TFMBN by fluorinating .alpha.,.alpha.,.alpha.-trichlorotolunitrile, but this fluorination reaction is difficult to complete so that the reaction product contains difluoro matter. DE-A No. 2,550,262 shows preparing TFMBN by reaction of trichloromethylbenzotrifluoride with ammonia, but the yield of the aimed compound is low: for example, less than 15% in the case of preparing 4-trifluoromethylbenzonitrile. Besides, in any of these methods complicated operations are needed for post-reaction treatment.
In general, as reported in J. Chem. Soc., (1965) 1564, nitriles can be prepared directly from aldehydes at fairly good yields by refluxing a solution of an aldehyde in formic acid with hydroxylamine. However, this method involves several problems from a practical point of view. First, recovery of formic acid is troublesome because the acid is diluted with by-produced water. Furthermore, the report mentions that the nitriles were isolated by diluting the reaction liquid with water. If the formic acid as diluted is reused in the reaction of the next batch the rate of reaction becomes very low. When this method is used for preparing TFMBN from trifluoromethylbenzaldehyde there arises another problem that isolation of the formed TFMBN is difficult because TFMBN and formic acid form an azeotrope. Besides, when brought into contact with water TFMBN relatively easily hydrolyzes into trifluoromethylbenzamide by reason of the electron effect of its trifluoromethyl group.